An inverter which serves for example at one end of an installation for high-voltage d-c transmission that feeds power to an adjoining a-c network, is frequently operated at a minimal quenching angle to provide an optimum active-power transport. As is well known, the quenching angle also known as the recovery time, must always be larger than a release time specified for the structural components of the valves (thyristors) used in the inverter. The minimum quenching angle is therefore a measure of the time period for which a negative anode-cathode voltage must be present at a particular valve after a current interruption before that valve can be stressed again by a positive blocking voltage. If this minimum quenching angle is observed, the valve remains in its current-blocking state and will not flip by itself back into a current-conducting state.
For adaptation to changed operating conditions, this minimum value of the quenching angle is frequently set by a quenching angle control. A method and apparatus for operation of a high-voltage d-c transmission system is described in German Published Unexamined Patent Application No. 33 40 540 in which the control times of a quenching angle control for the output of an HVDC long-distance transmission are greatly shortened by a pilot control for that output. In this manner, a continuous flip-proof operation of the inverter at an inverter step limit and at the same time at optimum efficiency are possible. An apparatus designated in that German Application as a "protection time control" for optimum utilization of an inverter-fed synchronous machine regarding the network and machine power factor is described in the paper by R. Saupe in Elektrotechnische Zeitschrift ETZ, Vol. 102, (1981) No. 1, on pages 14 to 18. (That paper relates to German Patent No. 307 221.) Instead of setting a constant minimum protection time value made out for the operating point with the largest permissible cos .rho., i.e., for maximum values of current and speed of rotation, the minimum protection time value is controlled in such a manner that the machine inverter always operates at the inverter flipping limit at an optimum cos .rho., as well as for heavy load-dependent changes of the actual quenching angle value.
In German Patent No. 307 221, the negative voltage present at each value for the duration of the protection time is blanked out, and a binary signal indicating its duration is digital-to-analog converted via an integrator followed by a sample-and-hold memory. With an additional device contained in the apparatus of German Patent No. 307 221, the negative voltages occurring at each individual valve (or thyristor) are determined directly and their duration in time is evaluated. To this end, voltages must be taken off at five points in a six-pulse inverter. These are the three phase voltages at the inverter output, and the intermediate phase to phase voltage at the inverter inputs as reference potentials for the phase voltages. This method for determining the actual quenching angle values via direct evaluation of the negative valve voltages has the disadvantage that in particular, two additional d-c voltage converters must be provided for determining the two potentials of the intermediate phase-to-phase voltage serving as reference quantities. Particularly in an inverter which is used in an HVDC system, the employment of d-c inverters at such a point is very costly due to the high d-c voltages on the transmission line. In addition, because of the unavoidable measuring inaccuracies of d-c converters, the quality of d-c voltage converters required by a quenching angle control can frequently not be achieved in this manner, and thereby, sufficient protection against undesired inverter flipping is not assured.
In another known method for determining the actual value of the quenching angle, the currents and voltages of the individual phases are determined. The rising flank of a binary signal indicating the quenching angle is formed directly via a current measurement by a conduction end signal when the valve current is quenched. The falling flank of the binary signal is formed by the subsequent zero crossing of the valve voltage determined via a voltage measurement. In this known method the additionally required a-c converters increase the measuring inaccuracy, especially due to the unavailable phase rotation of the converter output signals relative to the original signals.
Thus, the known methods have the problem of measuring inaccuracy in determining the actual value of the quenching angle, because of the need for further measurement variables at the inverter output.